It is also a synonym (in CatalanSenyal Reial or Senyera and old SpanishSeñal Real or Señera) for Royal Flag, although the word normally refers to the Catalan and Aragonese flags. Also in Aragonese, it is usually referred to as O Sinyal d'Aragón, i.e. "The Sign of Aragón".

The Senyera is one of the oldest flags in Europe to be used in present day, but not in continuous use since its creation. There are several theories advocating either a purely Catalan or a purely Aragonese origin for the symbol.

The Gran Enciclopedia Aragonesa states that its first material undisputed evidence dates back to a royal seal of Alfonso II of Aragon (1159), and that all evidence about a "Catalan" origin is debatable since historically nothing can be accepted other than the concept of "barcelonès", and understanding that as allusive to the House of Barcelona (Counts of Barcelona), and then, nothing referring to the ancient geographical area known as Cathalania, Catalonie (Catalonia)[2] where the County of Barcelona was found.

Furthermore, the Pennon of the Conquest of Valencia is documented as the world's oldest extant flag, dating from 1238, even though the yellow part was originally white, but the red stripes design was the same.

The Government of Catalonia states about its official symbols that it may derive from 11th century or 12th century pre-heraldic symbols and the County of Barcelona's coat of arms, in which the yellow and red bars were vertical, while horizontal in the flag.[3] The dynastic coat of arms became also the one for the lands ruled by the counts. It is thus present in the flags and shields of the territories that once were part of the Crown of Aragon and also in the arms of Andorra, Provence-Côte d'Azur and Sicily among others.

Another version is that the Kingdom of Aragon used and adopted the colours of the Papal States in their own coat of arms as a public and notorious submission to the Pope, something which the County of Barcelona would have followed shortly after according to this version. The colours for the city of Rome, which in those times was controlled by the Pope, were the same. According to the Encyclopædia Britannica, the flag of the Holy See's Navy from the 12th century on consisted of two vertical red and yellow bands, sometimes bearing the tiara and the keys. According to the Vatican official website, the yellow and red of the flag of the Holy See were two colors traditional of the Roman Senate and People.[4] This ancient flag can still seen on the Capitoline Hill in Rome, near the Roman Forum. The Papal States changed its colours in 1808 to the present yellow and white,[4] while the City of Rome sticks to the old colors to this day. We can reject this theory based on the works of Geronimo de Blancas, chronicler of the Kingdom of Aragon, 1585. This author reports that Pope Innocent III after crowning the King Pedro II (the Catholic) accepted that Aragon was a tributary kingdom of the Apostolic See. To compensate for this gesture the Pope appointed King Peter II confalonier of the Church and that the Church's banner had the colors of the crown of Aragon.

According to a 14th-century legend, the flag dates back from the 9th century, when the four red bars were drawn, as an act of gratitude, on Wilfred I the Hairy's (Count of Barcelona) golden shield by king Charles the Bald's fingers drenched with blood from the Count's war wounds prior to Wilfred's death in 897 during the siege of Barcelona by Lobo ibn Mohammed, the moor governor of Lleida. This legend would relate the emblem unambiguosly to the Counts of Barcelona title.[2]

Romantic-driven Catalan nationalists were particularly keen on this legend during the Renaixença, in the 19th century, albeit it has always been recognized and divulged as such even in patriotic circles. Charles the Bald had died 20 years earlier, in 877.

Another version of this legend cites Louis the Pious as the king drawing the bars during the conquest of Barcelona, in this version drawing them in a golden shield, but Louis died before Guifré was born.[7] Also, Barcelona was conquered long before the events described on the legend.[7] Another medieval variant of the legend features Ramon Berenguer painting the bars with his own blood on a yellow shield, with the yellow field of the shield being the arms of Aragon before his marriage.[8]

There are a few variations in the official flags of other territories. For instance, in Aragon an extra coat-of-arms, in Balearic Islands a castle in the canton, and in València a blue crowned fringe on the hoist.